Here is the solution for Mac. First let me restate the error message so it will make it easier for others to find this answer:
"Cannot Proceed: IPC Memory in use or image size is too big for the system."
In Affinity Photo, go to Preferences.
Choose Photoshop Plug-ins. (I assume you already added the Topaz Plugins. You should see them."
Near the bottom right, click the "Authorize Global" button.
You will have to restart Affinity. (It will put a path of / that represents the root directory.)
Your Topaz plugins should work now.
